1. Field of the Invention
The present invention relates to an autothermal aerobic digestion system for treatment bio-solid containing wastewater streams and industrial liquid organic wastewaters, etc., and more particularly, to an autothermal aerobic digestion system having closed defoaming system.
2. Description of the Related Art
Wastewater streams in the advanced society contain a variety of human derived bio-solid compounds. A variety of treatment has been developed over many numerous years for the treatment of bio-solid containing liquid streams since those sewage streams contain various contaminants derived from mostly humans. Those contaminants are generally of organic compounds and numerous kinds of microorganisms including pathogenic organisms that are harmful to humans. However, conventional method for the treatment of bio-solid containing wastewater streams frequently fail to get rid of these concerned pathogenic microorganisms such as E coli, Salmonella, enterovirus and certain protozoan organism, etc., even with heavy usage of chemicals, which sometimes entails unexpectedly casualty of innocent citizens. Accordingly, more efficient method for destroying pathogenic microorganisms such as so-called ATAD (Autothermal Thermophilic Aerobic Digestion) have been developed and applied in wastewater treatment and even fertilizer manufacture as well.
In addition to the chemical and/or biological removal aspect of pathogenic microorganisms, conventional wastewater treatment processes result in large amount of sludge that are dumped or land filled or sometimes sold as soil additives and fertilizer due to its nutrient aspects. However, the bigger wastewater treatment facility the more sludge generated causing handling and tricking problem, requiring more intensive management, raising liability issue in disposal and consequently entailing more financial expense. This sometimes requires extremely cost, since construction of whole treatment facility is necessary for bigger land and longer construction time. Therefore, since land is becoming more and more expensive in big metro city, special process and method have been developed with respect to better efficacy of using small land and quicker processing time of bio-solid containing wastewaters or bio-wastes from industries. However, peak time including particularly summer flood in the city forces the existing treatment plant to release wastewaters in partly untreated condition that expose public to dangerous health situation.
The sludge resulted from conventional wastewater treatment process is basically organic materials but also contains the same species of pathogenic bacteria as can be found in the untreated wastewaters as well. Those microflora include E coli, Salmonella and bacteria responsible for cholera, typhoid fever bacillary, brucellosis, etc. In addition, the sludge contains other group of thermophilic microflora that are not harmful to humans in nature and a certain group of thermophilic microorganisms. They could be aerobic or anaerobic organisms in nature. This invention relates with the use of thermophilic aerobic microorganisms.
The bio-solid or organic waste liquid streams in general term contain a variety of group of organic compounds that could be utilized or digested by aerobic microflora in bio-solid materials themselves that are of major organism in the system of this invention. Of the wide spectrum of aerobic organism are bacteria that thrive within thermophilic temperature ranging from 50° C. to 75° C. These organisms are generally not harmful to humans in nature. Of the variable group of organism in bio-solid materials are mesophilic microorganisms that are referred as pathogenic due to their active temperature range at about 37° C. that is around normal temperature of human body. The concern on the removal of these mesophilic organisms are becoming public issue and accordingly government regulatory body becomes more aware of regulations in terms of level of pathogenic organisms in the treated wastewater and the resulting sludge as well.
When bio-solid containing wastewater streams are treated in insulated reactor under heavy aerobic condition, as many others already discovered, the temperature of the reactor increases by exothermic reaction with oxygen. As temperature reaches mesophilic temperature range of 25° C. to 37° C., the pathogenic microorganisms begin to die, which is one of the major advantages of autothermal aerobic digestion system over conventional treatment method of bio-solid waste streams. The heat could be from external source such as electric heater and destroys all of the pathogenic organisms but it is very energy, cost intensive and not economical.
Another advantage of the autothermal aerobic digestion system is that the process generates itself heat without external heating source during autothermal process where the number of thermophilic microorganism grow and their exoenzyme activity increases, which eventually break down much of the organic compounds of the bio-solid materials in the waste streams at higher speed than conventional aerobic or even anaerobic treatment processes. The resulting amount of sludge becomes much lower than that produced by regular process mostly running at room temperature. The reaction of thermophilic anaerobic treatment system is even slower. Therefore, it could be concluded that the autothermal aerobic digestion system produces much less amount of sludge due to fast and efficient break-down of bio-solid materials to liquid status and reaches more than 99% of death rate of pathogen organism.
The reaction of the autothermal aerobic digestion system (or normally called ATAD system) in the industry of bio-solid containing wastewater stream treatment the thermophilic condition of the process generates foam. There could be theoretically and practically two types of foam in the digestion process; mechanical foam and biological foam. Mechanical foam relates with mechanical agitation of high viscosity waste streams caused by action of aerator or blender in the reactor. The fine bubbles by aerator of the thermophilic treatment process produce mechanical foam out of the liquid of viscosity at even ambient temperature before exothermal process takes over. In the mean time as exothermal reaction by thermophilic bacteria with the help of plentiful oxygen takes over in the reactor the temperature starts going up during which process a population of a certain group of microorganism at a certain temperature grow and become dominant in the reactor. However, as temperature continues to go up the particular group of microorganism starts dying away and another new group of microorganism takes over and becomes dominant again. This symptom continues to repeat until the temperature of the reactor reaches to its absolute limit of temperature when bio-solid nutrients of bacteria are consumed and become not available any more. Then exothermic reaction terminates and accordingly temperature drops as well.
Of the two types of foam is the biological foam that makes most of researches in wastewater treatment industry more interested since it causes troubles in actual operation. Proteins of dead bacteria generated through cell lysis that could not survive a certain temperature mostly cause the biological foam with help of mechanical agitation action by blender or aerator. Therefore, the biological foam forms at any time without warning sign depending upon the condition of thermophilic treatment process. Strength and amount of foam varies depending upon conditions such as types of bio-solid raw materials in the wastewater streams and their concentration, species of thermophilic microflora involved and temperature profile during the whole process.
The troubles caused by foam include 1) discontinuation of treatment process, 2) loss of bio-solid liquid streams through bypass, 3) contamination of surroundings, 4) shortened life expectancy of mechanical parts of reactor, 5) difficult situation to control major factors such as pH, temperature, oxidation-reduction value, aeration level, motor speed, etc due to the foam, 6) more extended period of whole process affecting adversely continuous automatic treatment system than scheduled.
Mechanical defoaming systems such as rotating blade and liquid defoamer using its own bio-solid liquid waste streams or even tap water have been introduced for years. The fundamental of those conventional defoaming apparatus is to physically rupture foam bubble that converts foam to liquid status. It is pretty effective with light and low amount of foam but not with heavy and strong foam. Therefore, the results are frequently considered unsatisfactory and those mechanical defoamers require high maintenance, high energy cost and more frequent examination of the apparatus. Particularly high maintenance such as periodic replacement of moving parts such as blade and motor, periodic procedure due to severe erosion problem is of major concerns.